Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, Tokyo 153-8902, Japan.
J Cell Sci. 2023 May 1;136(9). doi: 10.1242/jcs.261094. Epub 2023 May 9.
COPII proteins assemble at ER exit sites (ERES) to form transport carriers. The initiation of COPII assembly in the yeast Saccharomyces cerevisiae is triggered by the ER membrane protein Sec12. Sec16, which plays a critical role in COPII organization, localizes to ERES independently of Sec12. However, the mechanism underlying Sec16 localization is poorly understood. Here, we show that a Sec12 homolog, Sed4, is concentrated at ERES and mediates ERES localization of Sec16. We found that the interaction between Sec16 and Sed4 ensures their correct localization to ERES. Loss of the interaction with Sec16 leads to redistribution of Sed4 from the ERES specifically to high-curvature ER areas, such as the tubules and edges of the sheets. The luminal domain of Sed4 mediates this distribution, which is required for Sed4, but not for Sec16, to be concentrated at ERES. We further show that the luminal domain and its O-mannosylation are involved in the self-interaction of Sed4. Our findings provide insight into how Sec16 and Sed4 function interdependently at ERES.
COPII 蛋白在内质网出口位点 (ERES) 组装形成运输载体。酵母酿酒酵母中 COPII 组装的起始是由内质网膜蛋白 Sec12 触发的。Sec16 在 COPII 组织中起着关键作用，它独立于 Sec12 定位在 ERES 上。然而，Sec16 定位的机制还知之甚少。在这里，我们表明一种 Sec12 同源物 Sed4 集中在内质网出口部位并介导 Sec16 的内质网出口部位定位。我们发现 Sec16 和 Sed4 之间的相互作用确保了它们正确定位于内质网出口部位。与 Sec16 的相互作用丧失会导致 Sed4 从内质网出口部位重新分布到高曲率内质网区域，例如小管和薄片的边缘。Sed4 的腔结构域介导了这种分布，这对于 Sed4 而不是 Sec16 集中在内质网出口部位是必需的。我们进一步表明，腔结构域及其 O-甘露糖基化参与了 Sed4 的自我相互作用。我们的研究结果提供了对 Sec16 和 Sed4 在 ERES 上如何相互依存发挥作用的深入了解。
